In the formation of subterranean wells, a drilling fluid is circulated to the bottom of a borehole and ejected into the borehole from a drill bit. The drilling fluid usually returns to the surface of the well through the annulus of the borehole. At the surface, the drilling fluid is chemically and mechanically treated to provide to the drilling fluid the desired properties. Alternatively, the drilling fluid rises to the surface from the bottom of the borehole through a drill stem.
To perform the boring operation effectively, the drilling fluid must possess a variety of properties. For example, the drilling fluid should be capable of (1) transporting the cuttings, resulting from the drilling operation, out of the borehole; (2) cooling and lubricating the drill bit; (3) providing hydrostatic pressure to the borehole to prevent the hole from caving in or blowing out during the use of high pressure fluids; (4) providing a wall cake upon the borehole wall; (5) preventing the drilling fluid from losing excessive amounts of fluid; and (6) suspending the solid particles from the drilling operation when the drilling fluid ceases to circulate in the borehole.
Water based drilling fluids usually contain water and one or more additives or dispersants such as colloidal solids, non-colloidal solids, chemical dispersants, thinners, and dissolved salts. The properties of the drilling fluid may also be affected by the types of rocks through which the subterranean well is drilled. Usually, the viscosity, temperature, and pressure of the drilling fluid are altered as the borehole is drilled through a rock formation.
Various additives or dispersants have been proposed to provide the requisite properties in the drilling fluid. For example, bentonite, lignite, phosphate, and lignosulfonate containing materials are often added to the drilling fluid. The use of bentonite in the drilling fluid gives a thixotropic or gel structure to the drilling fluid.
Lignite is added to the drilling fluid to control the thixotropy of the drilling fluid. However, the presence of contaminants, such as sodium chloride, gypsum or anhydrite in the drilling fluid often renders the lignite ineffective as a deflocculent. Also, as the temperature and pressure of the drilling fluid increases, the lignite loses its effectiveness as a deflocculent.
Phosphate additives deflocculate the colloidal solids and thin the drilling fluid. However, the phosphate additives lose their effectiveness as the temperature and pressure of the drilling fluid increases.
Lignosulfonates have been also used as a drilling fluid additive, since they exhibit good deflocculating properties when used in calcium or salt containing drilling fluids. The lignosulfates, however, also degrade when the temperature of the well becomes relatively high. Since such high temperatures are often reached during the drilling of deep wells, lignosulfonates have limited usefulness. In contrast, polyacrylic acids exhibit excellent thinning and temperature stability properties, but they are sensitive to salts, such as calcium.
Consequently, an additive or dispersant is needed that is both thermally stable and resistant to degradation under various subterranean well conditions. Previous drilling fluid additives and dispersants have been ineffective in providing these desired properties to the drilling fluid. The previous additives and dispersants have limitations that lessen their usefulness in various borehole conditions, especially at a relatively high borehole temperature or pressure.
Some drilling fluid additives have been proposed to solve these problems. For example, U.S. Pat. No. 3,730,900 to Perricone et al. discloses an aqueous drilling fluid having a styrene sulfonic acid-maleic anhydride copolymer. Similarly, U.S. Pat. No. 3,332,872 to Oakes describes a drilling fluid having a copolymer of styrene and maleic anhydride. These copolymer additives, however, do not provide all of the requisite properties to the drilling fluid.